This invention relates generally to methods and apparatus for utilizing satellite signals in combination with an inertial navigation system (INS) to determine vehicle heading. More specifically, the invention relates to the use of signals transmitted by a Global Positioning System (GPS) satellite and INSs which utilize gyros with bias errors ranging up to one degree per hour.
Position, velocity, and attitude of a vehicle can be accurately determined using well-known methods of coupling GPS receiver/processors and INSs. Accurately determining the heading of a vehicle under all conditions of motion, however, requires either a long period of time gyrocompassing in the case of medium-accuracy INSs (i.e. gyro bias errors of .about.0.01 degrees/hour) or the use of a magnetic detector such as a flux valve in the case of low-accuracy INSs (i.e. gyro bias errors of .about.1 degree/hour).
INS measurements of heading can be corrected by utilizing GPS measurements of vehicle position and velocity when vehicle maneuvering is taking place. However, in the case of a low-accuracy INS, errors in heading during periods without turns or accelerations grow as a result of gyro drift--potentially one degree for every hour of non-acceleration--or be limited to about one degree by the errors in the flux valve.
Another approach is to use two GPS receivers having a common time reference with two antennas to derive the phase differences between GPS carrier phase as received by the antennas. The phase differences and knowledge of the GPS satellite positions as determined by their ephemerides and time can then be used with INS pitch and roll to determine vehicle heading. Because of the periodicity of the carrier phase, ambiguities arise and various methods have been devised to resolve them. The need for two receivers entails an undesirable cost penalty, especially when used with a low-cost INS.